A. Several things could cause it, but two things are more common than others. It could be some type of allergic reaction to something that's new in the environment - the skin around the mouth is pretty sensitive to things like that, and sometimes allergic reactions will result in swelling in that area.
The other possibility is some type of insect sting. Bees are really common culprits here, and dogs are famous for biting at them and getting stung. Hopefully it's not too serious, but do monitor for continued swelling and take her in if you see anything concerning, especially difficulty breathing or signs that the reaction is becoming more generalized (swelling or focal "wheels" in other parts of the body).
Also I'll just mention that I've seen dogs get bitten by snakes in the area around the mouth, so if you live anywhere where that's a possibility this time of year, definitely go right in to see a vet ASAP.
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